Flexible electrode materials have received great amount of interest due to their potential applications in wearable or roll up gadgets such as electronic papers, collapsible displays and other personal multimedia devices. Recent literature has introduced free standing paper carbon based electrodes that are promising for producing flexible electronic devices. Carbon nanotubes (CNT) and its composites have been extensively studied to form flexible electrodes. However, their relatively high production cost and difficulty of getting stable CNT dispersion have hindered its practical application.
Recently, graphene based electrodes have gained interest due to its remarkable mechanical and electrical properties as well as good electrochemical stability. In order to form flexible electrode, assembly of individual graphene nanosheets into a macroscopic freestanding and flexible graphene paper is of interest. Several recent reports on the freestanding graphene paper have shown good flexibility upon bending.
Flexible graphene based electrodes can be used in wide range applications such as thermoelectric, gas sensor, bio sensor, fuel cell, etc. Flexible graphene based electrodes can also be used for flexible energy storage devices such as batteries and supercapacitors. Although graphene based electrodes have been widely reported, most of the electrodes are in powder form that require metal substrates as their current collector or have a low mass (<0.5 mg cm−2) leading to a low areal capacitance (mF cm−2). High areal capacitance is crucial in miniaturization of the energy storage device for modern gadget applications. Electrodes that have high areal capacitance are able to store more charges compared to electrodes having a low areal capacitance.
Further improvement of flexible and free-standing graphene electrode is possible by incorporating pseudocapacitive materials to form flexible hybrid electrode. Among pseudocapacitive materials, ruthenium dioxide (RuO2) has been well recognized due to its high specific capacitance (up to 1300 F g−1) and good electrochemical stability. However, commercial application of RuO2 has been slow due high costs of RuO2. Thus, cheaper transition metal oxides, such as MnO2, V2O5, Co3O4 and NiO need to be further explored as the alternative electrode materials. The areal capacitance of hybrid metal oxide/graphene based flexible electrodes is still far from satisfactory. For instance, graphene/MnO2 coated on the textile has been reported with areal capacitance of 94.5 mF cm−2 (315 F g−1). A graphene/MnO2 paper electrode without any supporting current collector (i.e. textile, sponge, foam, metallic substrates) has also been reported. However, the mass of the electrode is very low (0.07 mg cm−2), thus the areal capacitance tends to be low, 17.9 mF cm−2 (256 F g−1).